Regulated power supply



Aug. 22, 1961 J. H. HERSHEY REGULATED POWER SUPPLY 3 Sheets-Sheet 1Filed Nov. l5, 1951 /NVE/VTOP J. HHRSHEV y 9 @LM/MN ATTORNEY llg- 22,1961 J. H. HERsHEY REGULATED PowEP SUPPLY 5 Sheetsj-Sheet 2 Filed Nov.l5. 1951 /Nl/ENTOR 5y J. H. HERSHEV o: SM

ATTORNEY Aug. 22, 1961 J. H. HERsl-u-:Y

REGULATED POWER SUPPLY 3 Sheets-Sheet 3 Filed Nov. l5. 1951 r.CN

a Q N m.. GF4.

/A/VE/vrof? HVJ. H. HERSHEY /9 ATTORNEY tes This invention relates tovoltage controlling apparatus and particularly to apparatus forsupplying to a space current device of fthe tunable magnetron typerecurring voltage pulses of regulated amplitude.

Magnetrons and, more particularly, tunable magnetrons are disclosed inan article in The Bell System Technical Journal, Volume XXV, No. 2,April 1946, entitled The Magnetron as a Generator of Centimeter Waves,by i. B. Fisk, H. D. Hagstrum and P. L. Hartman. In operating amagnetron, the power output of the device may be increased, within anoperating range, by increasing the direct space current supplied to theanode-cathode path of the device. Moreover, it has been found that asthe tuning of the magnetron is changed in a direction to increase thefrequency of the output wave, for example, the anode-cathode impedanceof the device increases substantially linearly with frequency so as todecrease the amplitude of the direct current llcWin-g in theanodecathode path when voltage puls of fixed peak amplitude areimpressed upon the magnetron. It has also been found, however, that theuseful life of a magnetron will be considerably shortened when operatedat a space current amplitude which is excessively high and also that theoperating efficiency of the device decreases as the peak space currentincreases beyond a certain maximum amplitude.

It is an object of the invention, therefore, to provide means forjointly controlling the tuning of a magnetron oscillation generator andthe voltage impressed upon the magnetron for energizing it to maintain asubstantially constant peak current through the space current path ofthe magnetron irrespective of the frequency of the wave generated by themagnetron.

A further object of the invention is to provide means for reducing thevoltage impressed upon a magnetron in response to misfit-ing or arcingbetween the anode and cathode of the magnetron.

In accordance with a specific embodiment of the invention herein shownand described for the purpose of illustration, there is provided forgenerating radio frequency waves a tunable, electromagnetic, spacecurrent resonator known as -a magnetron oscillator having a plurality ofcoupled resonant cavities and a suitable means for tuning the magnetronby varying either the inductance` or the capacitance of the resonantcavities, as described in the article in Bell System Technical Journal,supra. The tuning control mechanism of the magnetron may he driventhrough a flexible coupling shaft either by manual control or by a motorwhich is remotely controlled.

There is provided a regulated rectifying apparatus for producing a high,direct output voltage which is automatically maintained substantiallyconstant under certain fixed, normal operating conditions of themagnetron. Means are provided for deriving from the output voltage ofsaid regulated rectifier under said normal operating conditionsrecurrent, constant peak amplitude voltage pulses which are impressedupon the anode-cathode path of the magnetron to cause substantiallyconstant amplitude pulses of space current to flow Ithrough the spaceseparating the anode and cathode of the magnetron. Means are providedfor increasing or decreasing the output voltage of the regulatedrectifier and, therefore, the amplitude of lthe voltage pulses impressedupon the magnetron under control of the driving means for the tuningvten #l Patented Aug. 22, 1961 control mechanism of the magnetron. Themagnitude of the volt-age pulses impressed upon the magnetron is thusincreased in response to a frequency increase of the waves generated bythe magnetron and vice versa, so as to maintain the peak space currentof the device substantially constant irrespective of the frequency towhich the magnetron is tuned. In case the magnetron should repeatedlyarc or misiire, means are provided for reducing the voltage applied tothe magnetron to a low value and for subsequently causing the voltage torise over a relatively long period.

The invention may be better understood by referring to the followingdetailed description with reference to the accompanying drawing in whichFIGS. l, 2 and 3, when placed side by side in order with FIG. l at theleft, are a schematic view of a system for energizing a magnetron typespace current device embodying the invention.

Referring to the drawing, there is provided a high voltage, bridge-typerectifier 10 having a space current diode 11 in each of the bridge armsfor supplying current to a load when the rectier is suitably energized.The voltage measured between the output or load terminals 15 and 16 maybe of the order of 7.0 kilovolts, for example. Voltage dividingresistors 12, 13 and 14 in series are connected across `the output orload terminals 15 and 16, negative terminal 16 being grounded. A ripplefilter connected between the output terminals of bridge rectier 10 andthe output terminals 15, 16 comprises a choke coil 18 in series in thelead going from the positive terminal of the bridge rectier 10 to outputterminal 15, Condensers 19, 20, 21 and 22 and a resistor 23. Condensers21 and 22 in series are in a shunt path connecting terminals 15 and 16and Condensers 19 and 20 in series are in a shunt path connecting acommon terminal of the bridge rectilier 10 and choke coil 1S to ground.

Current from an alternating current supply source 25 Vis supplied to theinput terminals of rectifier 10 through a circuit comprisingtransformers 26 and 27, dual reactor 17 and gas-filled, grid-controlledspace current devices 28 and 29. The primary Winding of transformer 27is connected to alternating-current supply source 25 While the secondaryWinding of transformer 26 is connected to the input terminals ofrectifier 10. The cathodes of tubes 28 and 29 and a mid-terminal of thedual reactor 17 are connected to ground. Transformer 27 has twosecondary windings 30 and 31. During half-cycle periods of one polarityof the current from the supply source 25, the voltage across transformerWinding 30 causes current to flow through the space current path of tube28 and through the upper half of the primary of transform-er 26 so as toinduce a voltage of one polarity in the secondary of transformer 26.During half-cycle periods of opposite polarity of the current fromsource 25, the voltage across transformer winding 31 causes current toflow through the space current path of tube 29 and through the lowerhalf of the primary of transformer 26 so as to induce a voltage ofopposite polarity in the secondary of transformer 26. As will be furtherdescribed below, pulses are impressed upon the control electrode-cathodecircuits of tubes 28 and 29 alternately to initiate current conductionin the tubes, means being provided for shifting the phase of the pulsesto control the conducting periods of the tubes and, therefore, theaverage current owing in the primary of transformer 26. The voltageacross the secondary of transformer 26 is thus controlled to regulatethe rectifier output voltage at terminals 15, 16.

There is provided a tunable, multicavity magnetron oscillator 40, suchas described in United States Patent No. 2,459,030 to H. C. Jonas, J.l?. Laico and V. L. Ronci, January ll, 1949, comprising an anode 4 1, acathode 42, a cathode heater 43, a suitable tuning means 44, a permanentmagnet 45 and an output coupling device 46 for generating pulses of highpower, high frequency oscillations which are picked up by the couplingdevice 46 `and transmitted through a wave guide 47 to la suitableutilization apparatus, not shown. There is provided a motor 48 energizedby current from a suitable source 49 through a single-pole, double-throwswitch 50 or other suitable means for causing the motor to drive thetuning pins 44 through gearing 39 and internally threaded driving member38 in one direction for increasing the frequency of the oscillationsproduced by the magnetron or in the other direction for decreasing thefrequency. The motor 4S is also coupled to a shaft 36 through gearing 39and 37. There are provided a transformer 51, having a primary, asecondary and a tertiary winding, through which high voltage pulses areimpressed between the cathode 42 and the grounded anode 41 for causingspace current to flow through the device and a transformer 52 throughwhich alternating current from a supply source 53 is supplied to thecathode heater 43. The voltage across the secondary of transformer 52 isimpressed upon a circuit comprising in series the secondary and tertiarywindings of transformer 51, inductance coils 54, 55 and the cathodeheater 43, a coupling condenser 57 being provided in a path connectingthe common terminal of inductance coil 54 and the secondary oftransformer 51 and the common terminal of inductance coil 55 and thetertiary winding of transformer 51.

There are provided a pulse shaping, line network 60 comprising seriesinduetance elements 61 and shunt capacitive elements 62 and means forcharging and discharging the network similar to that disclosed in UnitedStates Patent No. 2,469,977 to H. Morrison, May 10, 1949. The network 60is charged through a circuit which m-ay be traced from the positiveterminal 15 of the high direct voltage source through an inductor 63,diode 64, the network 60, inductor 56 and the primary of transformer 51to the negative terminal 16. A condenser 24 is provided in a pathconnecting a mid-terminal of inductor 56 to ground. A gas-filled,grid-controlled space current device 65 is provided for discharging thenetwork 60 when the device is conductive through a circuit comprisinginductors 66 and 56, and the primary of transformer 51. The tube 65 ismade conductive intermittently by impressing triggering pulses from asource 67 upon the grid-cathode circuit of the tube 65 through a circuitcomprising a shunt condenser 69 and a series inductor 68.

When the voltage from terminals 15, 16 is applied to the circuit,network 60 begins to charge at a rate determined primarily by theinductance of reactor 63 and the combined capacity of condensers 62 ofnetwork 6i). Except for the fact that the diode 64 in the chargingcircuit conducts only in one direction, the charging current of network60 would be oscillatory, the voltage across the network reaching amaximum of nearly twice the voltage at terminals 15, 16. Since the diode64 prevents current lflow in the reverse direction, the network 60 willremain charged to said maximum voltage until discharged through the tube65 when made conductive. The circuit is designed so that the loadconnected to the terminals of network 60 is substantially equal to thecharacteristic impedance of the network. As a result, current owing inthe discharge circuit will cause the network voltage to be dividedequally between the terminals of the network and the load presented tothe network. Simultaneously, a wave will travel along the network, bereflected at the distant open circuited end and return to the inputterminals. During a period equal to about twice the transmission time ofthe network, the voltage across the load will be maintained at itsoriginal value. When the reflected voltage wave reaches the inputterminals of the network, the voltage across the network and the loadwill be reduced substantially to zero and, as a result, conduction intube 65 will be interrupted.

The network will then be charged again to repeat the cycle of operation.

When arcing occurs in the space current path of the magnetron, themismatch of the impedance of the network and that of the load caused bythe reduction of thev load impedance by the arc would result in aresidual charge on the network having a polarity such as to make theanode or tube 65 negative with respect tof the cathode, if no means wereprovided to prevent this. The `eiiect of such reverse polarity residualcharges would be to raise the voltage tor which the network is chargedduring tlre next normal [charging period and this eiect would becumulative from cycle to cycle. The resulting excessive voltage appliedto the magnetron would cause the arcing to continue and the life of themagnetron to be shortened. The charging of the network 60 in the reversedirection is substantially prevented by providing across the spacecurrent path of tube 65 and reactor 66 a path for con ducting current ina direction opposite from the conducting direction through tube 65, thisconducting path cornprising a diode 70 and resistors 71 and 72, all inseries. With this arrangement, any reverse charge in the network 61)will lind a low impedance path through diode 7i) for quickly dissipatingthe charge. Moreover, the voltage across resistor 72 due to currentowing through the diode 7i) is utilized to reduce the voltage acrossterminals 15, 16 in case the magnetron mistires or arcs repeatedly for apredetermined number of times, as will be described below, thus furtherprotecting the magnetron against arcing and increasing its useful life.

There is provided a blower 73 driven by a motor 74 when energized from acurrent supply source 75, thereby cooling the magnetron to maintain itsoperating temperature within safe operating limits. The blower isequipped with a magnetic pick-up unit 76 comprising a permanent magneticcore 78 with a soft iron wedgeshaped pole-piece 79 and a winding 77.This unit is mounted in a bracket, not shown, which places the polepiece79 in close proximity to the rotating blades Si) of the blower which maybe 24 in number, for example. When the blower is running at a speed of3450 revolutions per minute, for example, the changing lux through themagnetic circuit comprising the magnet 78 and the fan blades causes tobe generated in the winding 77 a voltage having a frequency of 1380cycles per second. This voltage remains nearly constant over a widerange of frequency which varies directly with the blower speed. Meansresponsive to the frequency reduction of this voltage which results whenthe blower slows down or fails to operate are provided for reducing theanode-cathode Voltage impressed upon the magnetron to thereby preventoverheating of the magnetron, as described below.

The remainder of the voltage supply system shown in the drawing isprovided for Kregulating the voltage supplied to the magnetron. There isprovided a space current device 32 having anodes 81 and S2, controlgrids 83 and 84 and a common cathode 85. Anode 81 is directly connectedto the positive terminal of a 30G-volt battery 86 while the anode 82 isconnected through a resistor 88 to the positive battery terminal. Thecathode is connected through a resistor 89 to the grounded negativeterminal of battery 86. There is provided a. second 30D-volt battery 87having its positive terminal grounded. A potentiometer 90 having avariable tap 91 is connected across battery 87. The variable tap 91 ofpotentiometer 99 is connected through resistors 92 and 93 to the grid 83and the grid is further connected by a condenser 96, to ground. A rstvoltage divider connected from the positive terminal of battery 86 tothe negative terminal of battery 87 comprises in series resistor 97,rheost-ats 98 and 99 and resistor 100, in order, a common terminal ofrheostats98 and 99 being conected to a common terminal of resistors 92and 93. A second voltage divider across batteries S6 and 87 comprises inseries resistors 88, 103 and 102. A resistor 103 and a condener 104 inshunt are provided in a path connecting the anode 82 to a voltagedivider comprised of series resistor 101 and shunt resistor 184 inparallel with condenser 185. The control grid 84 is connected to thevariable tap of a potentiometer 105. Potentiometer 105 is connected inseries with condenser 183 and a comon terminal of resistors 13 and 14providing a means for adjusting the phase margin of the voltageregulating loop.

There is provided a phase shifting bridge circuit having half portionsof the secondary winding of a translformer 106y in its first and secondarms, respectively, and a condenser 107 in its third arm, the primary oftransformer 106 being connected to the alternating-current supply source25. The space current path of a pentode space current 4device 108 iseiectively connected in the fourth arm of the phase shift bridge circuitthrough a bridge rectier 109 comprising four space current diodes 110,the rectier 109 being provided so that the anode of tube 108 will bepositive with respect to the cathode for both half-cycle periods of thealternating supply source 25. The common terminal of resistors 101 and184 is connected to the control electrode of tube 108. A voltageydivider comprising resistors 111 and 112 in series is connected acrossbattery 86 and the voltage across resistor 112 is impressed upon thescreen grid-cathode circuit of tube 108. The output terminals of thephase shifting circuit are connected to the primary Winding of atransformer 113. There is provided a voltage divider comprisingresistors 114 and 115 in series connected across a battery 123. A commonterminal of resistors 114 and 115 is connected to a mid-terminal of thesecondary winding of transformer 113 and the end terminals of thissecondary winding are connected through resistors 18S and 190 to thecontrol grids, respectively, of the triodes of a twin-triode spacecurrent device 116. The positive terminal of battery 86 is connectedthrough resistors 118 and 117 to the anodes ofthe triodes, the cathodesof the triodes being connected to ground. Resistors 117 and 118 are thusin the space current paths of the triodes, respectively, of tube 116.The control electrode-cathode circuit of the left-hand triode as viewedin the drawing, may be traced from the anode electrode of the right-handtriode through resistor 189 to the lefthand control electrode thencethrough resistor 18S through the upper half portion of the secondary oftransformer 113 to the common terminal of resistors 114 and 115. Thecontrol electrode-cathode circuit of the righthand triode may be tracedfrom the anode of the lefthand triode through resistor 191 to theright-hand control electrode thence through resistor 190 and the lowerhalf of the secondary transformer 113 to the common terminal ofresistors 114 and 115. The gas-filled tubes 28 and 29 are biased beyondcut-olf by a voltage equal to the difference of the voltage of battery123 and the voltage across resistors 119 and 120, respectively, aterminal of resistor 119 being connected through a resistor 121 to thecontrol grid of tube 28, a terminal of resistor 120 being connectedthrough a resistor 122 to the control grid of tube 29, the commonterminal of resistors 119 and 120 being connected to the negativeterminal of battery 123 and the cathodes of the tubes being grounded.The voltages across resistors 119 and `1.20 are set up by current frombattery 123 Flowing through resistor 119, a crystal rectier 192 and aresistor 124, in series, andby current from battery 123 flowing throughresistor 120, a crystal rectifier 193 and a resistor 125, in series. Theanode of the right-hand triode of tube 116 `isconnected through acondenser 126 to the common-terminal of resistors 124 and crystalrectifier 192 and the anode of the left-hand triode of tube 116 isconnected through a condenser 127 to the common terminal of resistors125 and crystal rectifier 193. The values of condensers 126 and 127 andof the resistors 124 and` 125 are selected to shift the phase byapproximately 45 degrees of the waves impressed upon the grid-cathodecircuits of tubes 28 and 29 for repeatedly initiating conductiontherein.

Considering the operation of the circuit as thus far described, if theoutput Voltage of rectifier 10 appearing across terminals 15, 16 shouldincrease, for example, the control grid4 84 of tube 32 will becomerelatively more positive with respect to the cathode S5, thereby makingthe control grid of tube 108 relatively more negative with respect toits cathode. The anode-cathode resistance of tube 108 in one arm of thephase shift bridge circuit is thus increased. The alternating componentvoltages impressed upon the grid-cathode circuits of the triodes of tube116 and, therefore, the alternating component voltages impressed uponthe grid-cathode circuits of tubes 28 and 29 are thus `delayed to causespace current conduction in tubes 28 and 29 to be initiated later in therespective halt-cycle periods when the anodes, respectively, arepositive with respect to the cathodes. In normal operation theinductance of the dual choke coil 17 provides an inductive load therebylimiting the rate of rise of the current when conduction is initiated intube 28, for example, and that tube continues to conduct until itsanode-cathode potential becomes zero. Delaying the starting ofconduction in tubes 28 and 29 has the elect of reducing the averagecurrent iiowing in opposite directions alternately through the primarywinding of transformer 26 and, therefore, of reducing the alternatingvoltage impressed upon the input of rectifier 10 from the secondary oftransformer 26. The output voltage of the rectier across terminals 15,16 is thus reduced to cause the initially assumed increase `of voltageat terminals 15, 16 to be minimized.

The motor 48 which drives the means 38, 44 for chang ing the tuning ofthe magnetron 40 also drives the movable contact 91 of potentiometer `90which is coupled directly to shaft 36. The voltage requirement atterminals 15, 16 :may be approximately 5.8 kilovolts `at the lowestmagnetron frequency `and 6.8 kilovolts lat the highest magnetronfrequency, for example. When the motor runs in a direction to increasethe frequency of the oscillations produced by the magnetron 40, thepotentiometer contact 91 is driven in a direction to make the grid 83 oftube 32 relatively more positive with respect to its cathode, therebyincreasing the current owing through cathode resistor 89 and making thegrid 84 relatively more negative with respect to the cathode. The-control grid of tube 108 is thus made relatively more positive withrespect to its cathode to decrease the anode-cathode resistance of tube108. The output voltage of the phase shifting circuit impressed throughtransformer 113 upon the grid-cathode circuits of tube 116 is thusladvanced in phase to cause the output voltage of rectifier 10 impressedacross terminals 15, 16 to be increased with the result that the voltagepulses impressed between the anode and cathode of the magnetron arecorrespondingly increased in amplitude so as to maintain the spacecurrent in the magnetron at a substantially constant amplitudeirrespective of `the change of tuning of the magnetron.

As previously stated, the voltage across resistor 72 due to currentflowing through the diode 70 is utilized to etfect a reduction of thevoltage at terminals 15, 16 in case the magnetron arcs or misfires tenor more times per second, for example, after which the Voltage atterminals 15, 16 will return to normal value over a relatively longperiod. If the magnetron continues to misre the voltage at terminals 15,16 will continue to be cyclically reduced and then increased. For thispurpose there is provided a circuit comprising space current devices130, 142 and 163.

The common terminal of resistors 71 and 72 is connccted to lthe negativeterminal of the crystal rectier 140. The positive terminal of thecrystal rectier connects to the common terminal olf resistor 139 andcondenser 141. The circuit from the negative battery 87 through resistor137 and potentiometer 138 provide an adjustable negative potential atthe variable contact of 138 which applied through resistor 139 fixes thelevel of negative pulses which will pass through the crystal rectifier140 and be applied through condenser 141 to grid terminal 133 of thedual triode 130. The dual triode 130 operates as a single shotmultivibrator in the following manner. in its quiescent condition thedual triode 130 has its 132, 134, 136 section cut oif by negative biassupplied to grid 134 from battery 87 through ristor 155 and at the sametime section 131, 133, 135 is conducting due to its grid 133 beinggrounded through resistor 156. Self-bias of this section is provided byresistor 151 connected between cathode 131 and ground. Anode voltageobtained from battery 86 is supplied to anodes 135 and 136 throughresistors 147 and 148 respectively. For multivibrator action anode 136is coupled to control grid 133 by condenser 149 and anode 135 is coupledto control grid 134 by resistor 153 and condenser 154 connected inshunt. With section 131, 133, 135 of the dual triode 139 conducting thepotential at the common point of resistors 153 and 155 is suiiicientlynegative to cut oif the space curren-t flow through resistors 148 and152. When a pulse of abnormal amplitude is `applied to the crystalrectifier 14) by an abnormal negative current through resisto-rs 71 and72, the negative pulse is coupled to lgrid 133 through condenser 141causing the space current in that section to be reduced. The reducedspace current causes -a positive signal to be applied to control grid134 resulting in space current flow between anode 136 yand cathode 132.This increase in space current lowers the potential of anode 136 whichis coupled through condenser 149 to grid 133 driving this section tocut-off. This condition is m\ain tained for about 300 microseconds whengrid 134 regains control and the circuit returns to its quiescent state.The switched interval is a function of resistors 150, 153, 155 andcondensers 149 and 154. .When space current ows through resistor 152 apositive potential at cathode 132 causes current to ow through resistor156, plate 143 to cathode 145 of dual diode 142 and partially chargescondenser 157. The potential across condenser 157 is a function of therate at which abnormal pulses are passed by the crystal rectifier 140with its peak potential adjustable by the variable arm of potentiometer161 connected to the diode-cathode 146. A voltage divider circuit isprovided by resistors 160 `and potentiometer 161 oonnected acrossbattery 87. A further dividing action is obtained using resistors 159and 158 connecting the common point between cathode 145 and condenser157 with the common connection of resistor 160 and potentiometer 161.The common point between resistors 158 and 159 connects with anodeterminal 144 of the dual diode 142, by-pass condenser 162, and thecontrol grid of pentode 163. In the absence of pulses at the input togrid 133 of the dual triode 130 condensers 157, 162 and the control gridof the pentode 163 are sufficiently negative as to cut olf lthe anodespace current of pentode 163. The anode of pentode 163 is connected togrid 83 of tube 32. The cathode of tube 163 is connected to ground andits screen is connected to the cornmon point of the voltage dividerresistors 164 and 165 connected across battery 86.

When an irregularity occurs in the load on transformer 51, negativecurrent pulses through resistor 72 cause abnormal negative pulses to beapplied to grid 133 of tube 130. Through the multivibrator action oftube 130 these short negative pulses result in constant amplitude pulseof ampliiied duration to be applied to condenser 157. When the number ofyabnormal pulses exceeds for example 10 per second, the increase inpositive potential of condenser 157 raises the potential on the controlgrid of pentode tube 163 causing space current to flow from the commonpoint of rheostats 98 and 99 through resistor 93 and the anode-cathodeof pentode 163. The space current flow through resistor 93 reduces thepotential at grid 83 of tube 32, thereby making the grid 83 relativelyless positive with respect to the cathode and reducing the current owthrough cathode resistor 89. The grid 84 is thus made relatively morepositive with respect to the cathode and the grid of tube 108 is maderelatively more negative with respect to its cathode to cause theanode-cathode resistance of tube 108 to increase. The alternatingvolt-age impressed from the phase shifting circuit to the grid circuitsof tube 116 is thus delayed to cause the voltage at terminals 15, 16 todecrease by a large amount. Current conduction in tube 163 will beinterrupted when the control grid voltage of the tube reaches cut-offand condenser 96 will recharge at a r-ate determined by its capacity andthe values of the resistors in the associated charging circuit and thevoltage at terminals 15, 16 will rise exponentially. vIf the arcing ofthe magnetron should continue, the charging of condenser 157 will berepeated to again cause .the voltage at terminals 15, 16 to be lowered.This cyclic decreasing of the voltage at terminals 15, 16 will continueas long as the arcing of the magnetron persists. When the yarcingceases, tube 163 will be maintained in its nonconducting state and thevoltage at terminals 15, 16 will be maintained at a substantiallyconstant value.

In case the blower 73 should fail to operate `at the required speed foradequately coo-ling the magnetron, means are provided for reducing theanode voltage applied to the magnetron to prevent overheating andresulting damage to the magnetron. For .this purpose there is providedla control circuit comprising a tetrode thyratron 177 and La slowrelease relay 178. There is provided a transformer 179 having 4a primarywinding 184) connected to an alternating-current supply source 181 andtwo secondary windings 182 yand 166. Current is supplied from winding166 to the cathode heater of tube 177, the grounded terminal of winding166 being connected to one terminal of winding 182 and the cathode oftube 177 being connected to one of the end terminals of winding 166. Aresistor 168 shunted by a condenser 169 is connected between the grid oftube 177 and the common terminal of'resistor 173 and condenser 171. Theanode-cathode circuit of tube 177 comprises the winding of relay 178shunted by a condenser 167 and winding 182 of transformer 179. Thegrid-cathode circuit comprises resistor 168 shunted by condenser 169,resistor 173 and winding 166. When the polarity of the alternatingvoltage impressed upon the anode-cathode circuit is such as to make theanode positive with respect to the cathode, the polarity of the voltagefrom transformer winding 166 impressed upon the grid-cathode circuit issuch as to make the grid negative with respect to the cathode. In theabsence of a voltage from winding 77, that is, when the blower is notrunnin-g, tube 177 cannot conduct since during each half cycle of thealternating current when the anode is positive with respect to thecathode, the control grid is made more negative with respect to thecathode.

When the blower 73 is operating at normal speed, a voltage of 1380cycles per second, for example, generated in the winding 77 is impressedupon the input of a highpass lter comprising series condensers and 171and a shunt inductive reactor 172, the output of the iilter beingterminated in a resistor 173 in series with winding 166 of transformer177. The alternating voltage at the output of the filter combined withthat from winding 166 is applied through condenser 169 and resistor 168in shunt to the grid of tube 177. The combined signal voltage thusimpressed upon the grid-cathode circuit is of sui'licient amplitude tocause conduction in tube 177. The relay remains operated due to thecharge on condenser 167 during intervals separating successiveconducting periods of tube 177, conduction in tube 177 being interruptedwhen the anode becomes negative with respect to the cathode. Thecondenser 169 shunted by resistor 168 in series with resistor 173maintains proper anode-grid phase relation during the turn-on interval.If the blower 73 should slow down so that the frequency of the voltagegenerated in winding 77 is reduced to 1150 cycles per second, forexample, the alternating voltage across resistor 173 will decrease, and,as a result, the combined alternating voltage will decrease sutiicientlyto stop conduction in tube 177 thereby deenergizing relay 178. Therelease of relay 178 will cause the voltage of battery 86 to be appliedthrough rheostat 94 to resistor 97 to make the grid 83 of tube 32relatively more negative with respect to its cathode, thereby making thegrid 84 of the tube relatively more positive with respect to the cathodeand the grid of tube 108 relatively more negative with respect to itscathode to increase the resistance of the space current path of tube108. The resulting delay ofthe voltage wave impressed upon thegrid-cathode circuits of tube 116 causes the voltage at terminals 15, 16and therefore, the anode-cathode voltage applied to the magnetron 40 tobe decreased. Overheating of the magnetron is thus substantiallyprevented. When the relay 171 is deenergized, current from battery 86 issupplied through a resistor 175 to an indicating lamp 176. Operation ofthe relay short-circuits the lamp 176 to interrupt its energization tothereby produce an indication that the blower 73 is running at normalspeed.

What is claimed is:

1. In combination, a rectifier having output terminals connected to aload, a irst transformer having a rst and a second primary winding and asecondary winding, said secondary winding being connected to inputterminals of said rectifier, a second transformer having a primarywinding connected to an alternating-current supply source and a rst anda second secondary winding, a iirst and a second gas-filled spacecurrent device each having an anode, a cathode and a control electrode,a circuit comprising said iirst primary and said first secondarywindings for connecting the anode of said first device to its cathode, acircuit comprising said second primary and said second secondarywindings for connecting the anode of said second device to its cathode,means for biasing each of said control electrodes negatively withrespect to its cathode, a third and a fourth space current device eachcomprising an anode, a cathode and a control grid, means for couplingthe anodes of said third and fourth devices to the control electrodes ofsaid Iirst and second devices respectively, means for coupling thecathodes of said rst, second, third and fourth devices, means 4forimpressing upon said anodes of said third and fourth devices withrespect to said cathodes a potential for causing the iiow of spacecurrent in said third and fourth devices, a third transformer having aprimary and a secondary winding, means for connecting the end terminalsof the secondary of said third transformer to the control grids of saidthird and fourth devices, means for connecting a mid-terminal of saidsecondary of said third transformer to said cathodes, means forsupplying current from said alternatingcurrent supply source to theprimary of said third transformer, and means for shifting the phase ofsaid current 10 supplied to the primary of said third transformer forcontrolling the voltage across said load. p

2. A combination in accordance with claim 1 in which said load comprisesa space current device and in which there is provided means responsiveto arcing in said space current device for controlling said phaseshifting means to reduce the voltage impressed upon said space currentdevice.

3. A combination in accordance with claim 1 in which said load comprisesa space current device having an anode and a cathode and in which thereare provided means for deriving from said rectifier output terminals andimpressing upon a circuit connecting said anode and said cathode a firstseries of voltage pulses for causing space current to ow in said device,means responsive to arcing in said space current device for setting up asecond series of voltage pulses one for each of the pulses of said firstseries, and means responsive to the occurrence within a predeterminedtime period of a plurality' of said pulses of said second series forcontrolling said phase shifting means to reduce the amplitude of saidpulses of said first series.

4. A combination in accordance with claim 1 in which there is providedmeans responsive to the output voltage of said rectiiier for controllingsaid phase shifting means for normally minimizing changes of outputvoltage of said rectifier. p,

5. A combination in accordance with claim 4 in which said load comprisesan electromagnetic resonator for generating electromagnetic waves havingmeans for changing the tuning of said resonator to change the frequencyof said waves and in which there are provided means for impressing uponsaid resonator for energizing it a voltage derived from said rectiiier,and means for controlling said tuning changing means and said phaseshifting means simultaneously to maintain a predetermined relationshipbetween the output voltage of said rectifier and the frequency of saidgenerated waves.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Glasoe et al.: Pulse Generators, MIT Radiation Series, vol.5, rst edition, 1948, pages 454 to 463.

Lab.

